Plainsong

The various services of the church were sung in plainsong. As
its name implies, it was simple, unadorned melody, solemn and
ritualistic. Its simplicity does not mean that it is lacking
in interest; the smoothly modulating lines of melody at times
achieve a remarkable beauty, and many of the more famous
plainsong chants have been used by composers as material for
more complex works. A particularly fine example is the
Mass for Four Voices by William Byrd.
(Go to the page where you can hear
it.)